Impression trays adapted to receive the impression material and to be pressed against the dentate mucosae to take an impression for a dental prosthesis are conventional and standardized in various sizes.
It is the dentist's job so to select the tray shape that corresponds to the shape of the jaw with the dental arch of which an impression is to be taken.
The viscosity of the impression material is, for instance, between 0.5 .times. 10.sup.6 to 1.2 .times. 10.sup.6 millipoise--i.e. the impression material is fairly rigid.
In conventional impression trays of this kind, the U-shaped trough cross-section of the envelope extends around the associated dental arch and the stationary mucosa at a distance from them of more than 5 mm and often as much as 10 mm or more.
Both the labial edge and the lingual or palatal edge terminate at irregular distances from the associated zones of the mucosae and have too much free space between the tray edge and the corresponding mucosa to be able to be of any use as a sealing strip. To this extent, the impression is, as it were, prepared with an open envelope.
When the known trays are used, zones arise in which relatively large volumes of impression material may be relatively passive whereas other parts of the impression are relatively thin. In the case of upper-jaw trays this applies more particularly to the gum region with which a special but flat and less pronounced palatal vault is associated in the known trays. This leads to disadvantages.
Often, the impression is not accurate enough in the whole--i.e., macroscopically speaking. Microscopically, it does not allow for precision modeling of the teeth and all the fine details of the jaw and gum. For instance, it fails to give a clear reproduction of gum pockets, topographical peculiarities of individual teeth, the shape of inlay preparations and other fine detail.
Both the upper-jaw and lower-jaw impressions prepared with known impression trays are insufficiently accurate.